This invention relates to chromium containing refractories and, more particularly, it concerns an improved burned refractory with low levels of soluble chromium.
Chromium in its various oxide forms is a useful constituent of refractories because of its high melting point and its relative inertness when in contact with corrosive slags, gases, and molten metals. The main forms of chromium used in refractories are the mineral chromite--a complex oxide consisting of about 30-60% Cr.sub.2 O.sub.3 --and pigment-grade chromium oxide (Cr.sub.2 O.sub.3). Under certain conditions, the acid form of chromium, chromic acid (CrO.sub.3), can form from these materials. After use, routine disposal of these refractories is of concern since CrO.sub.3 (chromium VI) can be dissolved by water. Simple leaching of used chrome-bearing refractories by rain water or ground water may dissolve CrO.sub.3 present in the refractory.
The U.S. Environmental Protection Agency has issued test methods to determine the level of water-soluble chromium from various materials and has specified a current (Yr. 1990) maximum allowable level of total water-soluble chromium (all valence states of chromium) to be no more than 5 mg/l (or 5 ppm). The test methods used to determine soluble chromium have evolved from the earliest method, the EP Method (Extraction Procedure), to the present method, the TCLP Method (Toxicity Characteristic Leaching Procedure--Method 1311).
Since the EPA guidelines were announced, producers and consumers of chrome-bearing materials have been involved with determining the level of soluble chromium in their products. It is now apparent that different products containing similar levels of total chromium can differ in their soluble chromium levels. Therefore, a good correlation between total chromium and soluble chromium is not immediately apparent.
In light of the foregoing, there is a need for an improved refractory and method having a decreased level of soluble chromium, and in particular, a burned refractory having a level of soluble chromium reduced to below 5 ppm.